1. Field of the Invention
The present invention relates to a radiation image recording and reproducing method, more particularly, to a radiation image recording and reproducing method utilizing a divalent europium activated barium fluorobromide stimulable phosphor.
2. Description of the Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and an intensifying screen. As a method replacing the above-mentioned conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phsophor described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. The radiation image recording and reproducing method involves steps of causing the stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically converting the emitted light to electric signals; and reproducing the electric signals as a visible image on a recording material such as a photo-sensitive film or on a displaying device such as CRT.
Examples of the stimulable phosphor employable in the above-mentioned radiation image recording and reproducing method include a cerium and samarium activated strontium sulfide phosphor (SrS:Ce,Sm), an europium and samarium activated strontium sulfide phosphor (SrS:Eu, Sm), an erbium activated thorium dioxide phosphor (ThO.sub.2 : Er), and an europium and samarium activated lanthanum oxisulfide phosphor (La.sub.2 O.sub.2 S:Eu,Sm), as disclosed in U.S. Pat. No. 3,859,527. Further, the above-mentioned U.S. Pat. No. 4,239,968 discloses an alkaline earth metal fluorohalide phosphor having the formula (Ba.sub.1-x,M.sup.2+.sub.x)FX: yA, in which M.sup.2+ is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd; X is at least one halogen selected from the group consisting of Cl, Br and I; A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er; and x and y are numbers satisfying the conditions of 0.ltoreq.x.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2, respectively.
In the above-described radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value, especially when the method is used for medical diagnosis.
The radiation image recording and reproducing method is, as described above, very useful for obtaining a radiation image as a visible image, but it is desired that the sensitivity of the method to the radiation is as high as possible to decrease the exposure dose. Especially when the radiation is applied to a human body as described above, the enhancement in the sensitivity of the method is of much value from the viewpoint of adverse effect of the radiation on the human body, even if the level of the enhancement is not so remarkable.
The sensitivity essentially depends on the intensity of stimulated emission of the phosphor employed in the radiation image recording and reproducing method. In general, the nearer the wavelength of stimulating rays comes to the peak wavelength in the stimulation spectrum of the phosphor employed in the method, the more the sensitivity of the method is enhanced, while the farther the wavelength of stimulating rays goes away from said peak wavelength, the more the sharpness of the image provided by the method is enhanced. Accordingly, it is desired to employ a stimulable phosphor showing stimulated emission of high intensity and to employ stimulating rays in combination with the stimulable phosphor, which have a stimulation wavelength to be as near as possible to the peak wavelength of the stimulation spectrum thereof, in order to obtain a high sensitive method.
For instance, a divalent europium activated barium fluorobromide phosphor (BaFBr:Eu.sup.2+ ; a phosphor of the above-mentioned formula disclosed in U.S. Pat. No. 4,239,968 in which x is 0), which is a representative one as the stimulable phosphor employable in the radiation image recording and reproducing method, gives light emission of high intensity (stimulated emission) with a peak wavelength of approximately 390 nm, and is of great value in practical use. It is heretofore known that the instant phosphor shows the maximum emission intensity at a stimulation wavelength of approximately 600 nm.
In practicing the radiation image recording and reproducing method utilizing the above-described phosphor, stimulating rays having a wavelength close to the peak wavelength of the stimulation spectrum thereof, namely approx. 600 nm, are employed to enhance to sensitivity, whereby reducing the exposure dose given to the object.
In the radiation image recording and reproducing method, a stimulable phosphor is generally employed in the form of a radiation image storage panel containing thereof. The radiation image storage panel comprises a support and a phosphor layer containing a stimulable phosphor which is provided on one surface of the support.
The radiation image storage panel employed in the method hardly deteriorates upon exposure to a radiation and stimulating rays, so that the panel can be used repeatedly for a long period. In practical use, however, after scanning the panel with stimulating rays (otherwise, in advance of next use of the panel), light in the stimulation wavelength region of the phosphor is applied to the panel so as to erase the radiation energy remaining in the panel, as disclosed in Japanese Patent Provisional Publication 56(1981)-11392 (corresponding to U.S. Pat. No. 4,400,619 and European patent publication No. 80103974.4). That is because the radiation energy stored in the panel is not completely released by scanning with the stimulating rays and a portion of the radiation energy still remains therein.
However, when the conventional divalent europium activated barium fluorobromide phosphor having such a stimulation spectrum that the emission intensity at the stimulation wavelength of 500 nm is lower than that at the stimulation wavelength of 600 nm, as described above, is employed, there is a tendency that the radiation energy remaining in the radiation image storage panel cannot be completely erased even by subjecting the panel to the above-described erasing procedure, and the unerased radiation energy remaining in the panel can be released upon exposure to stimulating rays with a lapse of time. This phenomenon is presumed to occur by the following reason.
When radiation energy is stored in the phosphor of the radiation image storage panel, a number of electrons in the phosphor are trapped in several trap levels, that is in the semi-stable state. Most of the electrons in the semi-stable state generally return to the ground state with light emission when exposing the phosphor to the stimulating rays and erasing light. On the other hand, a portion of the electrons are still kept in the semi-stable state even after the phosphor is stimulated with the stimulating rays and the erasing light, because these electrons are trapped in such trap levels as to be difficult in returning to the ground state by the stimulation. However, with a lapse of time the electrons kept in such trap levels transfer to the level to release energy easily by the stimulation, so that the electrons can return to the ground state with releasing light by the stimulation after a lapse of certain time.
In the case where the above-described phenomenon occurs, even if the radiation image storage panel is subjected to the erasing procedure, the radiation energy still remaining in the panel is released together with a radiation energy stored freshly therein in the subsequent use. This phenomenon is called "appearance of after-image". Since the appearance of after-image gives a noise in the subsequent use of the panel, it is required to avoid occurrence of such phenomenon as completely as possible. However, the sufficient prevention of the appearance of after-image can be hardly attained only by the conventional erasing procedure. Further, in order to prevent the appearance of after-image only by the erasing procedure, a complicated operaton such as repeating of erasing procedures is required, and these operations reduce advantages of the radiation image recording and reproducing method.
Accordingly, it is desired that the radiation energy still remaining in the panel after scanning with stimulating rays can be erased to such a level that the appearance of after-image does not substantially occur by the single erasing procedure. In other words, it is desired that all the radiation energy, or at least most of the radiation energy stored in the phosphor can be easily released upon exposure to a light within the stimulation wavelength region of stimulated emission of the phosphor.
Further, there arises another problem in the use of the aforementioned divalent europium activated barium fluorobromide phosphor in the radiation image storage panel.
For obtaining the radiation image stored in the radiation image storage panel as electric signals in the above-described radiation image recording and reproducing method, the light emission emitted from the radiation image in the panel is photo-electrically processed (or read out). This photo-electrical read-out procedure of the radiation image generally comprises a preliminary read-out procedure and a final read-out procedure. In the preliminary read-out procedure, the panel is scanned with a weak light to read out a portion of the radiation image stored in the panel, whereby determining the conditions for processing the signals to obtain an image having a suitable density and contrast in the final read-out procedure. In a control circuit to determine the signal processing conditions, several data such as a ratio between the intensity of laser beam employed for scanning in the preliminary read-out procedure and that in the final read-out procedure, a ratio between the amount of stimulated emission released from the panel in the preliminary read-out procedure and that in the final read-out procedure, and the like is beforehand input. In the following final read-out procedure, the panel is scanned with a strong light to read out the radiation image stored in the panel so as to obtain electric signals, and the electric signals are automatically processed in accordance with the previously determined conditions.
In the above-described procedures, the problem is that the aforementioned conventional divalent europium activated barium fluorobromide phosphor is liable to show a fading phenomenon. That is, the stimulation spectrum of the phosphor varies in its shape with a lapse of time after exposure to a radiation. The so varying stimulation spectrum results in greater reduction of the emission intensity especially in the wavelength region longer than 600 nm. Accordingly, the ratio between the amount of stimulated emission in the preliminary read-out precedure and that in the final read-out procedure is liable to vary as time goes by.
More in detail, since the ratio between the amount of stimulated emission (a read-out value) converted to electric signals in the above-mentioned preliminary read-out procedure and that in the final read-out procedure varies with lapse of time, the electric signals cannot be necessarily processed suitably and an image having a proper density and contrast cannot be obtained if the exposure time is not beforehand input in the control circuit, or if the read-out procedure is done at a time different from the time previously set for the final read-out. Accordingly, a stimulable phosphor employed in the radiation image recording and reproducing method is desired to show minimum variation of the ratio between the amount of emitted light in the preliminary read-out procedure and that in the final read-out procedure with a lapse of time.